As someone who does mostly coach domestic travel, I value a mile somewhere around one cent. I don't mind coach and probably wouldn't ever redeem an award for anything higher. The only way you'll see me in C or F is with an upgrade cert or an operational upgrade. If I ever get more ambitious in my travel, however, miles may be worth more to me...

However, expiring miles a la Southwest would devalue my miles almost to nothing. I don't fly or redeem awards enough to ever earn an award if the miles keep flowing out of my account!